The present invention relates to a hybrid structure comprising an inner metal liner (selected from the group consisting of pipes and pressure vessels) wrapped with and bonded to an outer layer of fiber-reinforced, cured polymer resin matrix composite. In another aspect the invention relates to a method for producing such a structure.
It is known to reinforce a steel liner, such as a pipe or vessel, by wrapping it with a layer of fiber-reinforced, cured polymer resin matrix composite. The product can be referred to as a xe2x80x98hybrid structurexe2x80x99.
The fibers can be selected from the group consisting of glass, carbon, graphite or aramid. The polymer resin can be selected from the group consisting of epoxy, vinylester, polyester, peek, nylon and polyethlene. The preferred combination is glass or carbon fibers in an epoxy resin matrix.
The hybrid structure can be formed in either of the following ways.
Layers of partially cured, pre-impregnated, fiber-reinforced tape or sheet can be sequentially applied to the liner. The innermost layer is bonded to the steel with structural adhesive. The layers are bonded to each other by a curing resin, positioned between them, which interacts chemically with the layer resin. This technique is disclosed in my published Canadian patent application No. 2,181,497.
An alternative technique involves drawing rovings of fiber through a liquid bath of resin and winding the resin-coated fibers onto the liner to form a wrapping. The wrapping is then cured in place. This technique is described, for example, in U.S. Pat. No. 4,559,974, issued to Fawley.
The present invention is based on the following discovery:
if a hybrid structure, comprising an inner metal liner, preferably steel, and an outer wrapping of layers of fiber-reinforced, cured polymer resin matrix composite (non-metallic), bonded to the liner and arranged in an angle-ply pattern, is internally pressurized, to cause the steel of the liner to yield while the composite remains elastic, and then is de-pressurized;
it will be found that, in the de-pressurized state, the steel will have a compressive residual stress while the composite wrapping will have a tensile residual stress;
with the result that, when subsequently re-pressurized, the elastic regime (that is, the linear part of the pressure versus strain curve) of the steel liner is extended and the liner will not yield at the previous yield pressure. Thus, it takes greater pressure to burst the steel liner.
This means that a thinner walled pipe or vessel, when wrapped and treated as described, can operate as safely in pressure service as a thicker walled pipe or vessel formed of the same steel. Otherwise stated, a lighter pipe or vessel can be modified to achieve the same pressure rating as a heavier pipe or vessel.
By way of further explanation, if one takes a hybrid pipe or pressure vessel prepared as described and subjects it to increasing internal pressurization, and plots a pressure versus strain curve, the following will be noted:
The curve will ascend linearly and with a steep slope, characteristic of the stiffness of the steel alone, until the point at which the steel yields. Over this interval, both the steel and composite undergo elastic deformation;
After the yield point of the steel, the curve continues ascending generally linearly, but with a lesser slope, characteristic of the composite. Over this interval, there is plastic deformation of the liner and elastic deformation of the composite. The steel now deforms more rapidly;
When the pressure is released, the composite wants to shrink back to its original diameter. However, the liner is now permanently deformed and can only shrink back to a diameter greater than its original diameter. As a result, the composite now is in tension and the steel is in compression.
When the so treated hybrid pipe or vessel is again internally pressurized and the pressure versus strain is plotted, a linear (elastic) response curve is produced up to the previously applied pressure.
By applying the technique of the invention, the fiber reinforced composite shares a substantial part of the applied load. In addition, the fatigue resistance of the hybrid pipe has been found to be improved because for a part of the operating pressure cycle the stress in the steel liner remains compressive. Because of these features, a lighter hybrid pipe treated in accordance with the invention has better mechanical performance than a hybrid pipe that has not been so treated or the liner alone.
In a preferred feature, the angle-ply pattern wrapping is provided in the form of a stack of sheets or layers of fiber-reinforced polymer resin matrix. The sheets are wrapped about the liner in what is referred to as an angle-ply pattern. That is, one sheet or ply is wrapped with its fibers at an angle of +xxc2x0 relative to the axis of the pipe and the next ply is wrapped with its fibers at an angle of xe2x88x92yxc2x0 relative to the axis. Most preferably the fibers of the sheets of the stack are arranged on the basis of +xcex8xc2x0, xe2x88x92xcex8xc2x0, +"igr"xc2x0. . . The phrase xe2x80x9cangle ply patternxe2x80x9d is intended to cover these variants. It is to be understood that the word xe2x80x9cwrappingxe2x80x9d is to be given a broad connotation. It denotes the cured covering, which may have involved winding sheets, tapes or fibers onto the liner.
The stack is bonded to the liner and the sheets are bonded one to another by curing the stack after it has been wrapped on the liner.
In one aspect the invention is directed to a fiber-reinforced hybrid structure comprising: an inner metal liner selected from the group consisting of pipes and vessels; and an outer wrapping of fiber-reinforced, cured polymer resin matrix composite, said wrapping being wrapped around the liner in an angle-ply pattern and bonded thereto; said structure having been internally pressurized, to cause the metal of the liner to yield while the composite layer remained elastic and then de-pressurized so that the liner has compressive residual stress and the composite layer has tensile residual stress after de-pressurization.
In another aspect the invention is directed to a method for making a reinforced hybrid structure comprising: providing a metal liner selected from the group consisting of pipes and vessels; wrapping the liner with a plurality of plies of uncured or partly-cured fiber-reinforced polymer resin composite arranged in an angle-ply pattern; curing the plies and bonding them to the liner and to one another to produce a wrapped liner; internally pressurizing the wrapped liner to cause the steel of the liner to yield while the composite remains elastic; and then de- pressurizing the wrapped liner to produce a reinforced hybrid structure in which the liner has compressive residual stress and the composite has tensile residual stress.